1. Field of the Invention
This invention relates to a device for measuring a light beam by means of a diffraction lattice (or grating). More particularly, the invention is concerned with such a device which is adaptable to a light metering device in a photographic camera.
2. Description of the Prior Art
The light metering device in a photographic camera splits a portion of an image-forming light beam which has passed through an object lens by a beam splitter, and the thus split light is detected by a light detector. For the light beam splitter useful for such a light metering device, there has so far been used a single half mirror which is obliquely provided in the camera. Such a beam splitting device for use in the camera, etc. should desirably be as thin as possible. U.S. Pat. No. 3,464,337 discloses a device in which two members each providing on its surface a plurality of slant reflective surfaces, are cemented together to form a prismatic line raster and part of the incident light is split in a geometrical-optics manner for light metering.
The present invention is to provide such a light metering device, but having a beam splitting device of much thinner thickness by the use of a diffraction lattice.
The characteristic which is desired of the diffraction grating in that case is that useless diffracted light, except a light beam having image formation which emerges from the diffraction grating and diffracted light of a specific order for light metering, is not created. The reason is that such diffracted light creates flare light or ghost image and is detrimental to the formation and observation of the object image.
A beam splitter which meets such a demand is disclosed in applicant's Japanese Laid-open patent application No. 42042/1978. The beam splitter disclosed therein employs a relief type diffraction grating of high mass-productivity.
As described in the specification of the aforementioned patent application, a diffraction grating by periodical concave-convex structure of its surface is usually called a relief type diffraction grating, and beam splitters utilizing such diffraction lattices can be mass-produced from a matrix by the use of a copy making technique such as compression or the like, and also permit utilization of materials such as various stable plastics known in this field of art, which leads to a cost advantage.
On the other hand, in the light metering device of a single lens reflex camera or the like, it is desired that light meterings different in light metering area, namely, average light metering in which the light from substantially the whole of the object field is metered and partial light metering in which only the light from a part of the object field is metered both be possible. Beam splitting devices which have achieved such a purpose are disclosed in U.S. Pat. Nos. 4,103,153 and 4,178,084. The devices disclosed in these patents comprise holographically prepared diffraction lattices of different areas superposed one upon the other, and independently take out split lights of different metering ranges corresponding to those areas. However, these plural split lights differ from each other in quantity of light detected by a detector, substantially in proportion to the area and therefore and, when the light metering system is changed in the same camera, it is necessary to re-adjust the output level.